Aesthetic water-quality parameter — color limit: For domestic water supplies, what is the maximum permissible color based on the cobalt–platinum (Hazen) color scale?

Difficulty: Easy

Correct Answer: 5 ppm (≈ 5 Hazen units)

Explanation:


Introduction / Context:
Color in drinking water is an aesthetic quality that influences consumer acceptance and indicates potential organic matter or dissolved metals. The cobalt–platinum (Hazen) color scale is a traditional reference for setting limits.



Given Data / Assumptions:

  • Public potable supply standards aimed at good appearance and taste.
  • Color measured against the Hazen scale.


Concept / Approach:
Most guidance sets a limit around 5 Hazen units for supplied water to avoid noticeable tint. While raw surface water may have higher color, treatment via coagulation, activated carbon, or oxidation is used to meet distribution standards.



Step-by-Step Solution:

Identify parameter: true color on Hazen scale.Recall common acceptability limit: ≈ 5 Hazen units.Choose the matching option.


Verification / Alternative check:
At values above ≈ 15 Hazen units, consumers typically notice visible tint; utilities target ≤ 5 for consistency.



Why Other Options Are Wrong:
10–25 Hazen units may still be safe health-wise but can trigger complaints and signal inadequate treatment.



Common Pitfalls:
Confusing color (Hazen) with turbidity (NTU); relying solely on chlorine for organic color removal instead of coagulation or adsorption.



Final Answer:
5 ppm (≈ 5 Hazen units)

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